X-ray absorption spectroscopy is an attractive technique for chemical analysis, for example, chemical oxidation state analysis. See, for example, McBreen, et al. “In situ time-resolved x-ray absorption near edge structure study of the nickel oxide electrode”, J. Phys. Chem. 93, 6308 (1989) and Shimizugawa, et al., “X-ray absorption fine structure of samarium-doped borate glasses”, J. Appl. Phys. 81, 6657 (1997), the disclosures of which are incorporated by reference herein. One specific form of x-ray absorption spectroscopy is X-ray absorption near-edge structure analysis, which is commonly referred to by its acronym “XANES” analysis. XANES analysis is a powerful, non-destructive technique for examining the chemical state of an element, for example, for examining the oxidation state of an element. Specifically, XANES analysis is an x-ray absorption spectroscopy technique in which the x-ray absorption coefficient of a sample is measured as a function of photon energy near the threshold of an absorption edge.
In a XANES measurement, the position of the x-ray absorption edge changes with oxidation state due to changes in the energy required to promote 1 s electrons to the continuum. In addition, fine structure both before and after the absorption edge can be used to measure details of the chemical structure of the absorbing element. For minor or trace constituents, details of the absorption edge can be determined by measuring the intensity of the characteristic x-ray fluorescence signal for that element as a function of the energy of the exciting x-ray beam in the region of the absorption edge.
The pre-edge region, or the region immediately below the absorption edge, contains valuable bonding information and the edge position contains information about the charge on the absorber. One characteristic of an absorber that can be determined by XANES is its oxidation state. For instance, the oxidation state of an absorber can be determined from the precise energy of the x-ray absorption edge and pre-edge features.
XANES analysis provides an effective method of determining the oxidation state of an element, for example, the oxidation states of chromium. For example, the K-edge XANES spectrum of [CrO4]2— is characterized by an intense pre-edge peak (see Bajt, et al., “Synchrotron X-ray Microprobe Determination of Chromate Content Using X-ray Absorption Near-Edge Structure”, Anal. Chem. 65, 1800–1804 (1993), the disclosure of which is incorporated by reference herein). Bajt, et al. demonstrated that quantitative analysis of trace level Cr(VI) can be achieved with the XANES technique using a synchrotron radiation source. In a recent study (see Dillon, et al. “Microprobe X-ray Absorption Spectroscopic Determination of the Oxidation state of V79 Chinese Hamster Lung Cells to Genotoxic Chromium Complexes”, Chem, Res. Toxicol 10, 533–535 (1997), the disclosure of which is incorporated by reference herein), the XANES technique performed with a synchrotron source also showed that the Cr oxidation state in animal lung cells can be determined.
Two modes of XANES analysis exist, namely, the transmission mode and the fluorescence mode. In the transmission mode, a XANES spectrum is obtained by measuring the intensity of the x-rays transmitted through a sample as a function of the energy of the incident x-rays, that is, the x-ray photons. In the fluorescence mode, the fluorescence signal of an absorber is measured as a function of the energy of the incident beam. For dilute biological systems, for example, the fluorescence method removes the ‘background’ absorption due to other constituents, thereby improving the sensitivity by orders of magnitude.
A major limitation of the application of the XANES technique has been that such measurements have only been carried out at synchrotron facilities where x-ray beams of the required intensity and bandwidth have been available. In many applications it is also desirable to have good spatial resolution since, for heterogeneous samples spot analysis on as small a scale as possible is crucial. Typically, the XANES technique requires an intense, monochromatic x-ray beam. In addition, in order to obtain the desired x-ray energy near the absorption edge of the absorption spectrum of a substance, it is preferred that the x-ray beam used in a XANES system also be tunable, that is, the energy of the beam is preferably variable and controllable. To obtain a tunable, monochromatic x-ray photons, a white x-ray source coupled with an x-ray monochromator is typically necessary. Conventional laboratory x-ray sources have very low conversion efficiency for continuum x-ray generation, that is, conventional laboratory x-rays sources are simply not bright enough. Conventional laboratory x-ray sources are also divergent and typically suffer intensity losses imposed by the inverse square law. Due to the low brightness and divergent nature of laboratory sources, many important applications of XANES must be done with a synchrotron radiation source that provides an intense, polarized white x-ray beam. However, the high cost, limited time of use, and typically remote nature of synchrotron sources are not suitable for routine laboratory analysis. Thus, there is a need in the art to provide an affordable, compact XANES analysis system for performing real-time chemical analysis, for example, real-time oxidation state determination.
Recent innovations in doubly-bent crystal fabrication technology makes the precise two-dimensional bending of crystal planes possible (see, for example, U.S. Pat. No. 6,285,506, issued Sep. 4, 2001, entitled “Curved Optical Device and Method of Fabrication”, which is hereby incorporated by reference herein). Doubly-curved crystals collect a large solid angle of x-ray photons from a diverging x-ray source and focus them to a relatively small spot with a narrow energy bandwidth. A high performance doubly-curved crystal can provide an intense monochromatic micro x-ray beam (see Chen, et al., “Microprobe X-ray Fluorescence with the Use of Point-focusing Diffractors”, Applied Physics Letters, 71 (13), 1884 (1997), the disclosure of which is incorporated by reference herein). The energy of the beam can be tuned within a certain range by scanning the crystal optic. Crystal optics provide high intensity gain and narrow energy bandwidth and are thus ideally suited for use in XANES analysis, for example, one aspect of the present invention employs a crystal optic, for instance, a doubly-curved, toroidal crystal optic, for directing and focusing x-rays in a XANES analysis system.
There are two principal types of focusing geometries for x-ray crystal optics, namely, the Johann geometry and the Johansson geometry. Three-dimensional point-to-point focusing geometries are obtained by rotating the Johann or Johansson geometry about the source-image line. The Johansson-type point-focusing geometry is free of geometrical aberration and provides a larger solid collection angle than the Johann type. But the Johansson geometry is very difficult to achieve in practice.